Numerous pollutant aerosols have particles smaller than 0.5 mm. Available theoretical and experimental evidence indicates that these small particles are deposited deeper in the lung than larger particles and that in this size range deposition increases with decreasing size. They are retained much longer than larger, more proximally deposited particles because the distal sites where they are deposited lack the efficient clearance mechanism of the large airways. Experimental deposition data for particles smaller than 0.5 mm is sparce, and much of what exists is of doubtful validity because of incomplete characterization of the aerosols used. No depositions data for particles smaller than 0.1 mm are available. The purpose of this study is characterize deposition of 0.01 to 0.5 mm aerosols in the human respiratory tract. The electrical aerosol size analyzer, capable of measuring aerosol size distributions for particles of 0.003 mm to 0.5 mm diameter and the single particle aerodynamic relaxation time analyzer, developed in this laboratory and capable of measuring size distribution for particles 0.1 mm to 10 mm in (aerodynamic) diameter will be used. The first phase of this study will consist of generating aerosols in the required size range, characterizing them, and charcterizing the aerosol inhalation system for use with there aerosols. The second phase will consist measurements in normal volunteers, which will be carried out by analyzing inhaled and exhaled aerosol to determine deposition. The effect of variations in tidal volume, respiratory rate, and mouth versus nose inhalation will be examined. The third phase will consist of deposition measurements in patients with pulmonary disease. Patients with obstructive lung disease, in whom previous deposition measurements have been limited to 0.55 mm or larger particles, and patients with restrictive lung disease, for whom no aerosol deposition measurements have been reported, will be studied. Deposition during spontaneous respiration as well as during respiration at specified tidal volumes and rates will be determined.